Aluminium alloys are based on aluminium, in which the main alloying elements are Cu, Mn, Si, Mg, Mg+Si, Zn. Aluminium alloy compositions are registered with The Aluminum Association. The aluminium alloys are divided into 9 families (Al1xxx to Al9xxx). The different families of alloys and the major alloying elements are:
- 1xxx: no alloying elements
- 2xxx: Copper
- 3xxx: Manganese
- 4xxx: Silicon
- 5xxx: Magnesium
- 6xxx: Magnesium and silicon
- 7xxx: Zinc, magnesium, and copper
- 8xxx: other elements which are not covered by other series
There are also two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable. Aluminium alloys containing alloying elements with limited solid solubility at room temperature and with a strong temperature dependence of solid solubility (for example Cu) can be strengthened by a suitable thermal treatment (precipitation hardening). The strength of heat treated commercial Al alloys exceeds 550 MPa.
Example – Aluminium Alloys – Series 2000 – Duralumin
Aluminium alloys of 2000 series are alloyed with copper, they can be precipitation hardened to strengths comparable to steel. Formerly referred to as duralumin, they were once the most common aerospace alloys, but were susceptible to stress corrosion cracking and are increasingly replaced by 7000 series in new designs. In addition to aluminium, the main materials in duralumin are copper, manganese and magnesium.
Duralumin (also called duraluminum, duraluminium, duralum, dural(l)ium, or dural) is a strong, lightweight alloy of aluminium discovered in 1910 by Alfred Wilm, a German metallurgist. He discovered that after quenching, an aluminium alloy containing 4% copper would slowly harden when left at room temperature for several days. This process is now known as natural aging. He also designed an alloy (Duralumin) suitable for strengthening by this process in what is now known as preciptitation hardening. Although an explanation for the phenomenon was not provided until 1919, duralumin was one of the first “age hardening” alloys used.
In terms of age hardening, solution annealed aluminium-copper alloys can be aged naturally at room temperature for four days or more to obtain maximum properties such as hardness and strength. This process is known as natural aging. At room temperature, the solubility of copper in aluminium drops to a small fraction of 1%. At this point, the copper solute is locked inside the aluminium lattice (matrix), but must “precipitate” out of the supersaturated aluminium lattice. The aging process also can be accelerated to a matter of hours after solution treatment and quenching by heating the supersaturated alloy to a specific temperature and holding at that temperature for a specified time. This process is called artificial aging.
Duralumin is relatively soft, ductile and easily workable under normal temperature. The alloy can be rolled, forged and extruded into various forms and products. The light weight and high strength of duralumin when compared to steel enabled its application in aircraft construction. Although the addition of copper improves strength, it also makes these alloys susceptible to corrosion. The electrical and heat conductivity of duralumin is less than that of pure aluminium and more than that of steel.
Example – Aluminium Alloys – 6061 Alloy
In general, 6000 series aluminium alloys are alloyed with magnesium and silicon. Alloy 6061 is one of the most widely used alloys in the 6000 Series. It has good mechanical properties, it is easy to machine, it is weldable, and can be precipitation hardened, but not to the high strengths that 2000 and 7000 can reach. It has very good corrosion resistance and very good weldability although reduced strength in the weld zone. The mechanical properties of 6061 depend greatly on the temper, or heat treatment, of the material. In comparison to 2024 alloy, 6061 is more easily worked and remains resistant to corrosion even when the surface is abraded.
This standard structural alloy, one of the most versatile of the heat-treatable alloys, is popular for medium to high strength requirements and has good toughness characteristics. Applications range from aircraft components (aircraft structures, such as wings and fuselages) to automotive parts such such as the chassis of the Audi A8. 6061-T6 is widely used for bicycle frames and components.
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